Wireless networks rely on a large number of individual cells to provide high capacity wireless services over large coverage areas such as market areas (e.g. cities), surrounding residential areas (e.g. suburbs, counties), highway corridors and rural areas. Continuous radio connectivity across these large coverage areas is accomplished via user mobility from one base station to others as the user traverses the network's operating area. High reliability mobility is an important aspect of mobile wireless networks in order to minimize the number of dropped calls or other abnormal discontinuation of radio service to the supported users.
A key feature of modern multi-base station mobility networks is the creation and maintenance of neighbor lists for each base station within the network. Each base station transmits its list of nearby neighbor cells to mobile devices such that a mobile device can continuously monitor the radio frequencies defined in the list and search for higher quality base stations to which it may handover if and when the mobile device experiences degraded signal quality from its current serving radio base station. In other words, during active call sessions, the mobile device continually monitors quality of its serving base station and measures signal quality of its current neighbor list searching for suitable quality handover candidates.
If the mobile device finds a higher quality signal coming from a defined neighboring base station during this scanning procedure and if it meets the criteria for triggering a handover, it initiates a handover request to the network. If the request is granted, the mobile device connects to the candidate base station in either a hard or soft handover mode depending on the particular radio network technology in question. If the original serving base station's signal quality drops below a defined signal quality threshold, the mobile device will be connected entirely to the new base station and the call will continue. Should the serving base station's signal quality degrade below an acceptable level prior to the mobile device scanning and locating a suitable high quality neighboring cell, the call will typically fail and the user will experience a disconnect from the system such as a dropped call.
Each base station maintains its own list of likely neighbor cells and communicates this list via over the air messaging to each mobile station within its coverage area. Mobile stations search this list repeatedly and frequently to support handover operations as described above. Automatic Neighbor Relationship (ANR) management functions can assist with the creation and maintenance of these neighbor lists.
In Long Term Evolution (LTE) and Long Term Evolution Advanced (LTE-A) networks, User Equipment (UE) can identify LTE cells based on the Physical Cell Identifier (PCI) that is included in the Primary Synchronization Signals (PSS) and Secondary Synchronization Signals (SSS) transmitted by base stations. The UE uses the PCI in the SSS and PSS in order to identify particular base stations on the UE's neighbor list.
A PCI is identified by decoding the PSS and SSS and adding the values together. The SSS is encoded with 168 physical layer cell identity group numbers, while the PSS is encoded with 3 physical layer identity numbers. Adding these two signals together yields a total of 504 PCIs. Because the number of PCIs is limited, PCIs are reused throughout a network.
In addition, operators may maintain a separate set of reserved PCIs, which can be used for small cells, testing, and other purposes. The reserved PCIs are not used for normal macrocell base stations, so the existence of reserved PCIs further limits the number of PCIs that are available in LTE networks.
Due to the limited number of PCIs in a given network, conflicts occur. UE must distinguish between different base stations for connection, synchronization and mobility purposes, but reuse of the same PCIs may lead UE to confuse cells that use the same PCI. When synchronization signals are sent at the same time, such as in Time Division Duplexing (TDD) systems, the signals may interfere, making it difficult for UE to properly decode a PCI.
Although proper planning can avoid many PCI issues, proper planning is not always followed. Cells may be added or have identifiers changed in manual process without using a planning tool, resulting in suboptimal PCI reuse. In addition, ANR processes may incorrectly identify cells as neighbors under some circumstances. Accordingly, cellular systems can benefit from technology that identifies and resolves PCI conflicts and collisions.